There is an increasing need for improved efficiency of internal combustion engines. In order to meet these needs and to comply with new emissions legislation, the operating pressure of diesel engines continues to increase and operating pressures of 3000 bar (300 MPa) are envisaged. However, these increased operating pressures present a variety of technical problems.
It is known to provide a fuel injection pump unit comprising a plunger operating within a barrel to raise fuel pressure before discharging the pressurised fuel to a high pressure manifold. However, known pump units are generally unsuitable for operating at the increased pressures now required. A prior art pump unit of this type is illustrated in FIG. 1 and described in detail below.
Known pump units typically rely on a combination of static and dynamic seals to seal the pumping chamber. However, due to the alternating pressure cycles encountered within the pump unit, even small inaccuracies in the manufacturing process may cause a seal to fail. For example, a high pressure static seal is typically provided to separate the low pressure supply gallery and the pressure chamber. The seal encounters cyclical pressure changes from very low to very high and, as a result of differential radial expansion, relative motion may be induced between the surfaces on each side of the seal interface. Even if the resulting motion is very small, fretting wear and failure may result.
Furthermore, the internal geometry of known pump units may include intersecting bores and these may result in high stresses being induced during operation. To ensure safe and reliable operation, the pump head may have to be formed from higher specification materials or specialised manufacturing processes used to reduce the operational stresses.
A further problem exacerbated by operating at high pressures is increased fuel leakage which may result in higher fuel consumption. The high pressures generated within the pumping chamber may result in radial expansion of the barrel. As there is no corresponding expansion of the plunger, fuel leakage past the plunger may result.
It is known from EP 12821861 to provide a fuel injection pump unit comprising a piston movable axially in a pump working space, a non-return piston and a shut-off piston. The non-return piston and the shut-off piston are both movable to engage each other during a compression stroke. Thus, the fuel injection pump requires that the pistons form a seal with each other and also with the housing to form a seal during the compression stroke. The requirement that the relative movement of both pistons is controlled is not ideal. Moreover, leakage from the pumping working space may be increased since two separate seals are required.
The present invention(s) at least in preferred embodiments attempts to overcome or ameliorate at least some of the problems associated with known pump units.